Patent GB-612436-A (GB)
Title: Improvements in and relating to automatic pilots
IPC: G01C19/00
Filed: 1946-03-13
Abstract: 612,436. Electric control systems. BRITISH THOMSON-HOUSTON CO., Ltd. March 13, 1946, No. 7876. Convention date, March 19, 1945. [Class 4] [Also in Group XXXV] An aircraft automatic pilot has normally balanced control devices which are unbalanced to perform a correcting function and synchronising means which balance the control devices prior to engagement of the automatic pilot. The directional gyroscope 4 controls a selsyn system 67, 68, the. output of which is fed to a balanced differentiating amplifier 34, the output varying in magnitude and sense with the movement of the gyroscope and being connected to a balanced, polarised relay 18 controlling switches 26, 27. These, in turn, control interacting relays 24, 25, which alternately operate and release owing to the change in potential which operation of either causes on the appropriate control grid of amplifying valve 38. However, the ratio of contact times is unbalanced by signals received from the gyroscope and the servomotor 9 runs in a direction dependent, on the greater contact time of either contact 16 or 17, and moves the rudder 1 accordingly. When both relays are released the armature is short-circuited to provide a. braking action. Follow-up is provided by a mechanical connection 77 between the rudder and the rotor of selsyn generator 72, the stator output of which is fed back into the input of amplifier 34 so as to tend to oppose the movement of the rudder. Similar selsyn arrangements 82, 83 and 87, 88, amplifiers 341 and 34<SP>11</SP>, and servomotors 631, 63<SP>11</SP> control the ailerons 2 and elevators 3, in response .to movement, of the outer and inner rings 81, 80 respectively, of a vertical gyroscope 5. Followup action is similarly introduced by selsyn transmitters 84 and 88 respectively. The synchronising means avoids violent movements of the control surfaces at the moment of engagement and comprises an arrangement by which the outputs of the amplifiers 34 are reduced' to zero, irrespective of, the attitude of the aircraft, prior to engaging the automatic pilot and is brought into use by setting the control switch 8 to " Synchronizing ". In this condition the A.C. and D.C. power is switched on, but engaging relay 33 is not energized, so that battery is disconnected from relays 24, 25. In the case of the rudder channel any displacement of the balanced relay 18, resulting from a discrepancy between the attitude of the aircraft and the position of the gyroscope, actuates change-over contacts, 94, 95, 96, 97 to drive a reversible motor 91, which moves the rotor of selsyn 68 in such a direction as to remove the unbalance. In the aileron and elevator channels closure of contacts 126, 127 of balanced relay 18 impresses either a predetermined voltage or a short circuit on the input terminals 108, 109 of trim signal generator 86, 90, whose A.C. output is fed back to the input of the amplifier. The circuit of a signal generator is shown in Fig. 6 and comprises a triode 110 in which the potential of the cathode depends on the potential across the input terminals 108<SP>1</SP>, 109 and which is biassed to work on the lower bend of its characteristic curve so that the amplitude of its A.C. output, iritroduced through resistor 115 from supply 42, 43, varies according to the magnitude of the D.C. input voltage. This output is 180 degrees out of the phase with the A.C. input of a second triode 111, to which it is connected, so that the A.C. output of this second valve is zero at a predetermined balance value of D.C. input, increases in one sense for higher values of input and increases in opposite phase for lower values. A resistance capacity combination 122, 123, retards the effect of rapid changes in input. In the embodiment described the output is zero when the input potential difference is six volts. Considering the aileron channel, unbalance of relay 181, Fig. 1b, in one direction causes a potential of 12 volts to be connected to the signal generator, while unbalance in the opposite direction short circuits the input terminals. In each case the A.C. output is in such a sense as to reduce the unbalance to zero and at the same time the passage of A.C. through the selsyn 83 turns the rotor to the corresponding position irrespective of the gyroscope settings. Identical operations occur in the elevator channel. When the control switch 8 is moved to the engaging position to operate relay 33, this breaks the connections between the amplifiers and synchronising gear and connects power to the servomotor relays 26, 27. Since the amplifiers 34 are already balanced and the selsyns 68, 83, 87 are positioned to give no output, there is no sudden movement of the control surfaces at the moment of cutting in and the gyroscopes take over stabilisation of the aircraft smoothly. Relay 33 also connects the input terminals of the pitch trim signal generator 90 to a pitch controller 153, manual operation of which impresses a controllable voltage on the said terminals. In the level flight position, this voltage is set at six volts so that the generator 90 is balanced. Variation' in either direction, however, gives an output which unbalances the amplifier'in the sense to give the desired change of attitude. The relay 33 furthermore makes changes which enable the aircraft to make controlled turns in response to movement of the turn controller 139. The spindle 140 of this controller moves the slider of a potentiometer 141 to connect a variable voltage of appropriate polarity to the armature of motor 91, to cause an unbalancing voltage to be fed from selsyn 68 to the rudder amplifier 34 and make the rudder move appropriately. At the same time, shaft 140 turns the rotor of selsyn 83 so that a predetermined voltage is fed to the aileron amplifier to give an approximately correct bank angle. Final correction is effected by a longitudinally mounted pendulum switch 145 which, if displaced from its central position, connects either 12 or zero volts, according to direction, to the input of the bank trim signal generator. The A.C. output in the appropriate sense is additionally superimposed on the amplifier input until the bank angle is adjusted to the correct value for the rate of turn and airspeed. Some form of speed controller 161 such as a flywheel or friction governor is coupled to the turn controller to prevent it being moved too quickly. Remote operation of the turn controller is also provided by a reversible motor 157 whose rotation may be controlled by selector switches 159 when an overriding switch 160 is appropriately set. The Specification also discusses the eliinination of gimbal error which arises from the fact that the outer gimbal ring 66 does not correctly indicate the direction of the gyroscope 64 during banked turns, this being comparable with the relative angular displacement which occurs between the two shafts of a universal coupling. It is deduced that a voltage must be introduced into the rudder amplifier input which is proportional to the square of the angle of bank, and is in such a direction as to oppose the voltage resulting from the error. This result is achieved by the coupled potentiometer arrangement 168, 169 which feeds A.C. to the rotor of selsyn 78, this rotor being driven at twice the speed of the selsyn 68.